1. Field of the Invention
The present invention relates to a solid oxide fuel cell and a fabricating method thereof, and more particularly, to a large area planar type high performance solid oxide anode-supported unit cell which can substantially reduce structural defects and interfacial defects.
2. Description of the Background Art
A fuel cell is a generator using electrochemical reaction of an oxidizer and a fuel. The fuel cell does not undergo a process of converting chemical energy of the fuel into thermo-mechanical energy, thereby improving power generation efficiency and environment preservation.
The fuel cells are classified into a Phosphoric Acid Fuel Cell (PAFC), a Polymer Electrolyte Membrane Fuel Cell (PEMFC), a Molten Carbonate Fuel Cell (MCFC) and a Solid Oxide Fuel Cell (SOFC) according to used electrolytes. The PEMFC has an operation range of about 80° C., the PAFC has an operation range of about 200° C., the MCFC has an operation range of about 650° C., and the SOFC has an operation range of about 800° C. The solid oxide fuel cell whose constituents are all made of solid phases such as ceramic and meal has the highest efficiency. In addition, the solid oxide fuel cell is advantageous in selection of various fuels and use of waste heat. Accordingly, the solid oxide fuel cell can be applied for cogeneration with gas turbine as well as household fuel cell.
The solid oxide fuel cell commonly employs ZrO2 as an electrolyte. Recently, yttria stabilized zirconia (YSZ) doped with Y2O3 has been mostly used. Various kinds of solid oxide fuel cells have been developed according to a unit cell configuration, a stack and an operating temperature. The unit cells are classified into an electrolyte-supported unit cell and an electrode-supported unit cell according to structural supports. The electrode-supported unit cells are classified into a cathode-supported unit cell and an anode-supported unit cell.
The anode-supported unit cell is fabricated by sequentially forming an anode functional layer, an electrolyte layer and a cathode layer on an anode support substrate. In fabrication of the anode-supported unit cell, surface defects of a porous anode support generate defects of an electrolyte. It is thus very important to appropriately control the pore structure of the anode support and to prevent large surface defects thereof.
In the porous anode prepared by using solid particles or polymer particles as a pore forming agent, resulting anode support substrate has bi-modal or tri-modal pore size distribution. When graphite is used as the pore forming agent, the shape of the pores has anisotropy, which increases process defects on the electrolyte layer. Coarse pores formed by multiplicity of the pore diameter distribution or anisotropy of the pore shape generate depressions or cracks on the electrolyte layer succeedingly formed on the anode by screen printing, thereby reducing the production yield and performance of the unit cell.
Another process defects generated in fabrication of the large area unit cell are delamination or cracks generated between the component layers. Such interfacial defects increase resistance of the unit cell, sharply deteriorate the performance of the unit cell, and decrease damage resistance to a thermal stress. The interfacial defects are generated due to differences in sintering shrinkage or thermal expansion coefficient between the component layers. When the interface strength is weak, the interfacial defects usually increases in size, which reduces the production yield and deteriorates performance of the unit cell in operation. When the thermal stress is generated, the lifespan of the unit cell is seriously shortened. The interfacial defects of the unit cell of the solid oxide fuel cell mostly result from structural defects on the surface of the anode, powder packing non-uniformity of the anode functional layer and/or the electrolyte layer which is formed in thick film by successive screen printing, and low interface adhesion strength to the electrolyte layer of the cathode layer having a functionally graded structure (gradient-given structure in microstructure or property, for example, porosity of the electrode layer is reduced from the outside to the inside).
Especially, when a thermosetting binder is used to fabricate the anode support to obtain a uniform porous structure, segregations and coarse pores of the thermosetting binder worsen wettability of a paste for forming a thick film or generate depressions on a printed thick film during the process of forming the anode functional layer or the electrolyte layer by screen printing. Such defects may result in defects of the anode functional layer or the electrolyte layer and the associated interfaces. The problems occurring in the process of forming the thick film by screen printing make it more difficult to fabricate a high performance unit cell having a very thin electrolyte.